1. Field of the Invention
The present invention relates to ceramic granules for producing sintered products of silicon nitride that can be favorably used as structural materials for various heat engines such as automotive parts, gas turbines and the like, and as abrasion resistant materials and corrosion resistant materials like aluminum melt. The invention further relates to a process for preparing the ceramic granules and to a process for producing sintered products of silicon nitride by using the ceramic granules and, particularly, to a process for producing sintered products of silicon nitride that can be favorably used as parts that require high dimensional precision.
2. Description of the Prior Art
Owing to their excellent strength, heat resistance, shock resistance, abrasion resistance, corrosion resistance and light weight, sintered products of silicon nitride have been used as engineering ceramics and, particularly, as structural materials for heat engines such as turbo rotors and the like components, and as abrasion resistant members. Since the sintered products of silicon nitride exhibit difficultly sintering property, however, oxides of elements of the Group 3a of periodic table such as Y.sub.2 O.sub.3 and Sc.sub.2 O.sub.3 as well as aluminum oxide have been used as sintering assistants for preparing sintered products having high density and large strength.
In order to produce a sintered product having a high dimensional precision while suppressing contraction due to firing, furthermore, there has been proposed in U.S. Pat. No. 4,351,787 a process in which a mixture powder of a metallic silicon powder and a sintering assistant is formed, the mixture powder is heated in nitrogen at a temperature of not higher than 1300.degree. C. to nitrogenate the metal silicon into the silicon nitride, and is then heated at a temperature of not lower than 1600.degree. C. to increase the density. Japanese Patent Publication No. 24789/1990 discloses a process for nitrogenating and sintering a molded article comprising a metallic silicon powder, a silicon nitride powder, an oxide powder of an element of the Group 3a of periodic table and an aluminum oxide powder.
As the methods of molding the ceramics, furthermore, dry-type pressurized molding methods (inclusive of metal mold press method, rubber press method, rolling method, etc.) have heretofore been employed as convenient mass-production methods.
The dry-type pressurized molding method include a molding method in which the mixed starting materials are directly pressurized and are molded, as well as a method in which the coagulated granules (secondary particles) are prepared from the mixed starting material powder, and the granules are pressurized and smashed to prepare a molded article. In particular, the latter method is much used for the mass production since it features excellent powder filling performance permitting continuous molding. As a method of preparing such granules, a spray-drying method is now most generally used according to which a slurry suspension obtained by dispersing the mixed starting materials in a suitable solvent is spray-dried.
According to the spray-drying method in which the suspension on the slurry is sprayed into a container of a high temperature to form granules by quick drying, however, it is not possible to control the density of particles in the granules arousing a problem in that dispersion takes place among the production lots. Furthermore, the granules prepared by the spray-drying method that are pressurized during the molding under the application of pressure, are less smashed and form space among the granules. Therefore, large voids are formed in the sintered product causing the mechanical properties to be varied.
When a sintered product of silicon nitride is prepared through a step of nitrogenating the metallic silicon powder in nitrogen, the nitrogenation takes place based upon a reaction between the metallic silicon powder in the molded article and the nitrogen gas in the atmosphere. To prepare a sintered product having excellent dimensional precision, high density and large strength, it is essential that the metal silicon powder in the molded article is completely nitrogenated and the molded article after nitrogenated has a high density and homogeneous texture. When the mixed starting materials containing the metallic silicon powder are simply molded under the application of pressure to obtain a molded article, however, the molded article after nitrogenated acquires a low density and exhibits a decreased dimensional precision though the metallic silicon may easily react with the nitrogen gas during the nitrogenation. On the other hand, when the mixed starting materials are granulated to form ceramic granules and are molded, there can be obtained a molded article having a relatively high density. Depending upon the properties of the ceramic granules, however, the voids and density of the molded article undergo a change after it is molded under the application of a pressure, making it difficult to stably obtain a sintered product having excellent properties.